Dry-eye syndrome is a common condition affecting approximately one in five Americans. Dry eye is characterized by a decrease in tear production or an increase in tear film evaporation, together with the ocular surface disease that results. Symptoms include dry, irritated eyes, excessively watery eyes, burning and stinging, a foreign body sensation, and blurred vision. Despite the diverse causes of dry eye syndrome, in all dry eye conditions the ocular surface epithelium undergoes squamous metaplasia, manifested by loss of goblet cells, mucin deficiency and keratinization. These changes result in tear film instability, which leads to the clinical symptoms of dry eye syndrome.
Dry eye syndrome typically results from deficiency in the quality or quantity of tears produced by the patient. Precorneal tear film has traditionally been considered to have a three-layered structure. The closest to the cornea lies the mucin, or mucus, layer. The mucin layer provides an interface between the corneal epithelium and the remainder of the tear film. Overlying the mucin layer is the watery aqueous layer, which is the thickest layer of the three. The outermost layer of the precorneal tear film is the lipid layer. The lipid layer is an oily film that reduces evaporation from the aqueous layer beneath it.
The middle aqueous layer provides moisture to the corneal tissue, carries important nutrients, and serves to remove metabolic waste produced by the cornea. Deficiency in any of the three layers of the precorneal tear film can result in complaints of dry, gritty feeling or burning eyes.
The mucin that forms the mucin layer, nearest the cornea, is secreted by goblet cells in the conjunctiva. The conjunctiva is the transparent tissue that covers the sciera and the backside of the eyelids. The mucin layer functions to decrease surface tension of the tear film. In addition, the cornea itself is hydrophobic. Without the mucin layer to provide a bridge between the cornea and the aqueous layer, the aqueous layer would bead up and allow dry spot formation on the cornea.
The aqueous layer is secreted primarily by the glands of Wolfring and Krause located in the eyelid margin. The aqueous layer helps provide an optically smooth, transparent surface to the precorneal tear film. The lipid layer is secreted by the meibomian glands, and the glands of Zeiss and Moll. The glands of Zeiss and Moll are also located at the eyelid margin.
Currently, the pharmaceutical treatment of dry eye disease is mostly limited to administration of artificial tears (saline solution) to temporarily rehydrate the eyes. However, relief is short-lived and frequent dosing is necessary. In addition, artificial tears often have contra-indications and incompatibility with soft contact lenses.
Further, the use of medications, such as, 5-methyl-isoxazole-4-carboxylic acid anilides, hydroxyethylidene-cyano acetic acid anilide derivatives, and 2-(4-chlorobenzoylamino)-3-(2-quinolon-4-yl)propionic acid for the treatment xerophthalmia syndrome (“dry eyes”) has been attempted with limited results.
Another common disorder of the eyes is “vitreous opacities,” commonly known as “eye floaters” or “specks in the eye.” They are annoying visual disturbances, often accompanied by flashes of light which are caused by deposits of various size, shape, consistency, refractive index, and motility within the eye's normally transparent vitreous humour. They may be of embryonic origin or acquired due to degenerative changes of the vitreous humour or retina. The perception of floaters is known as myodesopsia, or less commonly as myiodeopsia, myiodesopsia, or myodeopsia. When observed subjectively, floaters are entoptic phenomena characterized by shadow-like shapes which appear singly or together with several others in one's field of vision. They may appear as spots, threads, or fragments of cobwebs, that float slowly before one's eyes.
Floaters are not uncommon, however, floaters are more than a nuisance and a distraction to those with severe cases, especially if the spots seem to constantly drift through the field of vision. The shapes are shadows projected onto the retina by tiny structures of protein or other cell debris discarded over the years and trapped in the vitreous humour.
Treatments include, among others, vitrectomy for more severe cases, however, the procedure is typically not warranted in those with lesser symptoms due to the potential for complications as severe as blindness. Another treatment is laser vitreolysis. This procedure can be time consuming and there is no consensus as to how completely effective it is.
Due to the complex pathologies of “dry eye” syndrome and “vitreous opacities,” current therapies, e.g., eye drop preparations and the like, provide only temporary medical measures, thus the need for effective medical treatment of the symptoms of “dry eye” and “vitreous opacities” still remains.